High-voltage fuse having composite fusible element structure

ABSTRACT

A current-limiting fuse having a composite fusible element structure which includes a wire section for interrupting overload currents, and a ribbon section having series necks for interrupting major fault currents, or short circuit currents. The wire section is straight and is connected in series with the ribbon section and the ribbon section is formed by a strip of zig-zag-shaped relatively hard and resilient silver forming a self-supporting extension spring tending to maintain the wire section under axial stress.

United States Patent Inventor Frederick J. Kozacka 7 South Hampton, NJ-I. Appl. No. 82,330 Filed Oct. 20, 1970 Patented Dec. 28, 1971 Assignee The Chase-Shawmut Company Newburyport, Mass.

HIGH-VOLTAGE FUSE HAVING COMPOSITE FUSIBLE ELEMENT STRUCTURE 9 Claims, 4 Drawing Figs.

US. Cl 337/161, 337/292 Int. Cl ..1-l0lh 85/04 Field of Search 337/159,

[56] References Cited UNITED STATES PATENTS 2,439,674 4/1948 Schuck 337/293 3,571,775 3/l97l Kozackam. 337/295 X Primary Examiner Bernard A. Gilheany Assistant Examiner-F. E. Bell Altomey- Erwin Salzer ABSTRACT: A current-limiting fusehaving a composite fusible element structure which includes a wire section for interrupting overload currents, and a ribbon section having series necks for interrupting major fault currents, or short circuit currents. The wire section is straight and is connected in series with the ribbon section and the ribbon section is formed by a strip of zig-zag-shaped relatively hard and resilient silver forming a self-supporting extension spring tending to maintain the wire section under axial stress.

INVENTOR FREDERICK J. KOZACKA FIG.2

1 1 I. r- I PAIENTED [E828 lsn FIG.I

HIGH-VOLTAGE FUSE HAVING COMPOSITE FUSIBLE ELEMENT STRUCTURE BACKGROUND OF INVENTION This application relates to a further development, or improvement, of the fuses disclosed in the copending patent application of Frederick J. Kozacka et al., filed Mar. 3, 1970, Ser. No. 16,1 16 for HIGH-VOLTAGE FUSE HAVING HELI- CALLY WOUND FUSE LINKS. The aforementioned patent application discloses a current-limiting high-voltage fuse having a composite fusible element structure which includes a wire section for interrupting overload currents, and a ribbon section having series necks for interrupting major fault currents. The wire section is straight and is connected in series with the ribbon section and the latter is formed by a strip of relatively hard resilient silver which is wound helically and forms a self-supporting extension spring tending to maintain the wire section under axial stress. In this design the width of the ribbon forming the ribbon section is subject to limitations which result from the need to maintain a specific spacing between adjacent turns of the helically wound ribbon. This need stems from the fact that, the fusible element structure must have a given length, that there must be a certain voltage gradient along the helical portion thereof which cannot be exceeded, that short-circuiting of contiguous turns by fulgurites formed incident to blowing must be avoided and that the helical extension spring calls for acertain loading to be reasonably self-supporting. These constraints place a limit on the currentcarrying capacity, orcurrent rating, of each composite fusible element structure. The development of a current-limiting fuse in accordance with the above-referred-to copending patent application resulted in composite fusible element structures whose ribbon sections had a width of 0.086 inch and a current rating of 12-E amps. Because of this relatively small current rating of each composite fusible element structure, high-current-carrying-capacity fuses designed according to the above patent application require a large number of parallel connected fusible element structures. This increases the bulk and the manufacturing cost of such fuses.

The principal problem underlying the present invention is to provide fuses with composite wire andribbon-fusible element structures having the same performance characteristics as fusible element structures of the kind disclosed in the above patent application Ser. No. 16,1 16, but having a much larger current-carrying capacity than those disclosed in the above patent applicating.

SUMMARY OF INVENTION Fuses embodying this invention include a tubular casing of insulating material closed on the ends thereof by a pair of electroconductive terminal elements. There is a pulverulent arcquenching filler inside of the casing. A composite fusible element structure arranged inside the casing, conductively interconnecting the pair of terminal elements is submersed in the arc-quenching filler. The composite fusible element structure has a straight wire section of circular cross section and a ribbon section of relatively hard and resilient silver provided with series necks and connected in series with the straight wire section. The ribbon sectionis folded in transverse direction at a plurality of equidistantlyspaced points and forms a zig-zagshaped extension having a predetermined length in nonloaded condition and an increased length in leaded condition, said spring being arranged in loaded condition between said pair of terminal elements and being self-supporting and tending to maintain said straight wire section under axial stress.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a longitudinal section of a current-limiting highvoltage fuse embodying the present invention taken along 1- l of FIG. 2, a portion of the structure of FIG. 1 situated between the terminal elements thereof being broken away;

FIG. 2 shows in front elevation'a portion of the composite fusible element structure of FIG. 1 and is drawn on a larger scale than FIG. I;

FIG. 3 shows the entire fuse structure of FIG. 1 in more diagrammatic fashion than FIG. I and on a smaller scale than FIG. I; and

FIG. 4 is a section along IV-IV of FIG. 1 and is drawn on the same scale as FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the drawings, numeral 1 has been applied' to indicate a tubular casing of electric-insulating material, preferably a melamine-glass-cloth-laminate. The electroconductive terminal elements are formed by a pair of terminal plugs of metal press-fitted into the ends of easing l and secured therein by transverse steel pins 3 projecting through casing 1 into terminal plugs 2. The axially inner end surfaces of terminal plugs 2 are provided with grooves 2a. The latter extend from the centers of plugs 2 radially outwardly and enclose predetermined equal angles. In the embodiment of the invention shown these angles are (FIG. 4). Casing l is filled with a pulverulent arc-quenching filler 4, preferably quartz sand, which is free from impurities. The fuse includes four fusible element structures 5, arranged inside casing l, conductively interconnecting terminal elements or terminal plugs 2 and submersed in arc-quenching filler 4. These four composite fusible element structures 5 are arranged in a substantially cylindrical pattern around the axis of easing I. Each fusible element structure 5 includes a straight-wire section 5'of circular cross section and a ribbon section 5" of relatively hard and resilient silver provided with series necks. The term neck, as used in this context, is intended to refer to portions of ribbon sections 5" of reduced cross-sectional area. In the embodiment of the invention shown the necks, or points of reduced cross-sectional area, are formed by circular perforations provided in ribbon sections 5". Wire sections 5' and ribbon sections 5" are connected in series and the latter are folded in transverse direction at a plurality of equidistantly spaced points at an acute angle to form a zig-zag-shaped extension spring. The spring has a predetermined length in nonloaded condition. Its length is increased when it is in loaded condition. Extension springs 5" are arranged in loaded condition between terminal elements or plugs 2 and are then sufficiently stressed to be self-supporting and tending to maintain straight wire section 5' under axial stress. Each wire section 5' is formed by a plurality of silver wires which are connected in parallel. The number of constituent wires of each wire section 5' should be at least three, and preferably more than three. Each wire is provided adjacent its center with a globule of soft solder, or other alloy-forming, silver-severing metal having a lower fusing point than silver. Each wire section 5' is arranged between a pair of zig-zag-shaped extension springs 5". The axially inner ends of zig-zag-shaped extension springs 5" are affixed by spot-welds to the axially outer ends of wire sections 5". Each spot-weld is covered by an overlay 6 of soft solder, or other alloy-forming, silver-severing metal having a lower fusing point than silver. Each ribbon section 5" includes, or is made up, of two ribbon subsections 5" of unequal length. All wire sections 5' have the same length, or are of equal length. In the embodiment of the invention shown the ends of two relatively long ribbon subsections 5" are inserted with the axially outer ends thereof into grooves 2a of the upper terminal plug 2, and the axially outer ends of two relatively long ribbon subsections 5" are inserted into the grooves 20 of the lower terminal plug 2. The axially outer ends of two relatively short ribbon subsections 5" are inserted into grooves 2a of the upper terminal plug 2, and the axially outer ends of two relatively short ribbon subsections are inserted into grooves 2a of lower terminal plug 2. Each axially outer end of a ribbon section, or subsection, is held in position within its groove 2a by means of a soft solder joint situated within the respective groove. Each wire section 5' is positioned between a relatively short ribbon subsection and a relatively long ribbon subsection. As a result, two wire sections 5' are displayed relative to the remaining two wire sections 5' in a direction longitudinally of easing l.

As shown in FIG. 2 the two ribbon subsections 5" are conductively interconnected by four wires forming wire section 5. Each wire has a globule 6 of soft solder at the center region thereof. The axially outer end of the wires which form wire section 5' are spot-welded to the axially inner ends of perforated ribbon extension spring 5" and the spot-welds by which the wires are attached to ribbon extension springs 5" are covered by overlays 6' of soft solder capable of severing the aforementioned wires and a aforementioned extension springs 5" by a metallurgical reaction involving metal interdiffusion and alloy formation upon fusion of the overlays 6. The circular perforations p provided in ribbon subsection 5" form points of reduced cross-sectional area and ribbon subsections 5" are folded in transverse direction at each of their circular perforations and stretched and form a pair of zig-zag-shaped loaded springs having planar elements enclosing an angle in the order of 90 degrees. The angular relation of the planar elements of ribbon springs 5 is best illustrated in FIG. 1, but also clearly shown in FIG. 3.

As clearly shown in FIGS. 1 and 3, a terminal cap 7 is mounted on each end of easing 1 and terminalcaps 7 are secured to terminal plugs 2 by means of centrally arranged hex screws 8. The later clamp the axially inner end surfaces of terminal caps 7 against the axially outer end surfaces of terminal plugs 2, thus establishing a good electrical contact between caps 7 and terminal plugs 2.

In FIG. I the letter L has been applied to indicate the axial spacing of the axially inner end surfaces of terminal plugs 2, the letter W has been applied to indicate the length of the straight wire section 5' and R, and R have been applied to indicate the length of the ribbon sections 5" in the leaded condition thereof. Rlbbon sections 5" have in their nonloaded condition the lengths r and r R r l and R, r,,.

(W+ r,+r L (l) (W+R,+R (2) n occurrence of relatively small overload currents of inadmissible duration wires 6 are severed by a metallurgical reaction at the points where the globules 6 of soft solder are located. This results initially in the formation of a single break in each of the four parallel connected silver wires As a result, the temperature of the silver wires undergoes a sudden increase. The metal-severing overlays 6' melt on account of this sudden increase in temperature. Thus three series breaks are formed in each of the four parallel-current paths formed by straight silver wires 5. This results in insertion of substantial arc resistance into the current path formed by the fuse. Additional resistance may be inserted into the current path of the fuse by back-burn of ribbon subsections 5". Since wire sections 5' are arranged out of registry or displaced relative to each other in a direction longitudinally of the casing l of the fuse, substantially equal cooling duties are imposed upon all portions of the quartz sand filler 4.

The melting t values of the wire sections 5' and of the ribbon sections 5" are unequal. Therefore the ribbon sections 5 and the wire sections 5" melt sequentially on occurrence of major fault currents. The sequence of their melting is determined by the magnitude of their F1 values.

The spring action of ribbon sections 5"is sufficient to maintain the wire sections 5' substantially straight during the assembly of the fuse structure. The loading of springs 5" is so large that the composite fusible element structures 5', 5" are entirely self-supporting any time prior to filling of the casing 1 with the arc-quenching tiller 4. This is best done through one of the central apertures, or holes, in terminal plugs 2 which receive the cap-clamping hex screws 8 after the fuse structure is fully assembled. After the fuse casing 1 has been filled with the pulverulent arc-quenching filler 4 the latter supports the composite fusible element structure 5, yet the spring action of parts 5" remains still useful since it tends to avoid thermal stresses and fatigue. During the process of assembly and filling the spring action of parts 5" tends to keep parts 5" in alignment with each other and in alignment with wire section 5'. Fuses according to the present invention compare in many respects favorably with conventional high-voltage fuses including a common insulating mandrel of relatively large diameter supporting a plurality of spaced helically wound-ribbon fuse links. One of these favorable distinctions resides in the relative increase of space inside the casing available for pulverulent arc-quenching filler, which relative increase is due to the absence of the link-supporting mandrel. Another favorable distinction relates to the S/L ratio, wherein S is the length of the zigzag shaped spring ribbon section 5" and L" the useful length of the casing of the fuse. In high-voltage fuses including a common-insulting mandrel of relatively large diameter supporting a plurality of spaced helically wound ribbon fuse links the S/L ratio decreases as the number of fuse links on the common mandrel increases. This is due to the fact that there must be a minimum axial-spacing between adjacent turns of fuse links on the common-insulating mandrel; As a result of this requirement the angular pitch of each fuse link must be increased, as the number of fuse links is increased. This increase of angular pitch results in a decrease of S/L ratio. Under such circumstances the S/L ratio may be increased, or improved, by arranging inside of a common casing a plurality of zig-zag-shaped fuse link extension springs.

For the purpose of this invention ziq-zag-shaped fuse links must have certain elastic properties, ie they must operate like extension springs and have a certain dimensional stability when suitably loaded. To this end they must be made of sheet silver having a high elastic or Youngs modulus or, in other words, of relatively hard silver. SIlver known as hard silver is preferred. It is, however, possible to fold ribbons of half hard silver into zig-zag-shaped springs satisfactory for the purposes of this invention.

The silver ribbons to be formed into springs ought to be rectangular in cross section and, as mentioned above, have serially related points of reduced cross-sectional area.

The success of the filling operation of a fuse according to this invention depends primarily upon three factors:

a. the arrangement of the springs 5" in a cylindrical pattern;

b. the orientation of the jet of sand filled into the fuse-easing strictly in the direction of the axis of that pattern; and

c. the ability of the springs 5" to withstand limited transverse forces without undergoing any significant distortion.

The greater the ability of springs 5" to withstand limited transverse forces without significant distortion thereof in the absence of any lateral support thereof, the less rigid the.

requirement to orient the jet of quartz sand strictly in the direction of the axis of the cylindrical pattern formed by springs 5" must be enforced. On the other hand, if the ability of springs 5" to withstand small lateral forces without distortion is relatively limited, then the requirement of proper sand jet orientation must be met rigorously.

It will be understood that l have illustrated and described herein a preferred embodiment of my invention, and that various alterations may be made therein without departing from the spirit and scope of the appended claims.

I claim as my invention:

1. A high-voltage fuse including a. a tubular casing of electric insulating material;

b. a pair of electroconductive terminal elements closing the ends of said casing;

c. a pulverulent arc-quenching filler inside said casing; and

d. a composite fusible element structure arranged inside said casing, conductively interconnecting said pair of terminal elements and submersed in said arc-quenching filler, said composite fusible element structure having a straight wire section of circular cross section and a ribbon section of relatively hard and resilient silver provided with series necks and connected in series with said straight wire section, said ribbon section being folded in transverse direction at a plurality of equidistantly spaced points and forming a zigzag-shaped extension spring having a predetermined length in nonloaded condition and an increased length in loaded condition, said extension spring being arranged in loaded condition between said pair of terminal elements and being self-supporting and tending to maintain said straight wire section under axial stress.

2. A high-voltage fuse as specified in claim 1 including a plurality of composite fusible element structures arranged in a substantially cylindrical pattern around the axis of said casing.

3. A high-voltage fuse as specified in claim 1 wherein said ribbon section is formed by a pair of aligned ribbon subsections spaced in a direction longitudinally of said casing, wherein said straight wire section is of silver and arranged between said pair of ribbon subsections with the axially outer ends thereof affixed to the axially inner ends of said pair of ribbon subsections, said pair of ribbon subsections jointly tending to maintain said straight wire section under axial stress, wherein said straight wire section is provided with a pair of low-fusing point silver-severing means each adjacent one of said ends thereof affixed to said pair of ribbon subsections, and wherein said straight wire section is provided with an additional low fusing point silver severing means affixed to said straight wire section substantially midway between the ends thereof.

4. A high-voltage fuse as specified in claim 1 including a pair of composite fusible element structure, the ribbon section of each of said pair of fusible element structures comprising a pair of aligned ribbon subsections of unequal length spaced in a direction longitudinally of said casing, said straight wire section of each of said pair of composite fusible element structures being of equal length and arranged between said pair of ribbon subsections thereof, one relatively short and one relatively long of said ribbon subsections of said pair of composite fusible element structures being arranged immediately adjacent to one of said pair of terminal elements and the other relatively short and the other relatively long of said ribbon subsections of said pair of composite fusible element structures being arranged immediately adjacent to the other of said pair of terminal elements so that said straight wire section of one ofsaid pair of composite fusible element structures is displaced relative to the said straight wire section of the other of said pair of fusible element structures in a direction longitudinally of said casing.

S. A high-voltage fuse as specified in claim 1 including:

a. a straight wire section formed by a plurality of silver wires connected in parallel and each having a deposit of a lowfusing point silver-severing metal adjacent the center thereof; and

b. a ribbon section formed by a pair of aligned ribbon subsections spaced from each other in a direction longitudinally of said casing and conductively interconnected by said plurality of silver wires, each of said pair of ribbon subsections having equidistantly spaced circular perforations, being folded at each of said plurality of perforations in transverse direction and stretched to form a pair of zigzag-shaped loaded springs having planar elements enclosing an angle in the order of 90 degrees.

6. A high-voltage fuse as specified in claim 1 wherein said straight wire section is formed by a plurality of silver wires connected in parallel, each having a deposit thereon of a lowfusing point silver-severing metal arranged substantially midway between the ends thereof, and said plurality of silver wires including at least three silver wires.

7. A high-voltage fuse including:

a. a tubular casing of electric insulating material;

b. a pair of electroconductive terminal plugs press-fitted into the ends of said casing and closing said ends, each of said pair of terminal plugs having grooves in the axially inner end surfaces thereof;

c. an arc-quenching filler of quartz sand inside said casing;

and

d. a plurality of composite fusible element structures arranged inside said casing and submersed in said arcquenching filler, each of said plurality of composite fusible element structures having an axially inner wire section formed by a plurality of silver wires connected in parallel composite fusible element structures further having a pair of aligned axially outer ribbon sections spaced in a direction longitudinally of said casing and conductively interconnected by said axially inner wire section, each of said axially outer ribbon sections being of relatively hard and resilient silver and folded in transverse direction at a plurality of equidistantly spaced points and forming zigzag-shaped extension springs having a predetermined length when unloaded and an increased length when in axially loaded condition, said extension springs being arranged in axially loaded condition between said pair of terminal plugs and the axially outer ends of said extension springs being inserted into said grooves in said axially inner end surfaces of said pair of terminal plugs and maintained in position therein by solder joints.

8. A high-voltage fuse including:

a. a tubular casing of electric-insulating material;

b. a pair of electroconductive terminal elements closing the ends of said casing and having axially inner surfaces having a predetermined axial spring;

c. a pulverulent arc-quenching filler inside said casing; and

d. a composite fusible element structure inside said casing,

conductively interconnecting said pair of terminal elements and submersed in said arc-quenching filler, said fusible element structure having a straight wire section and a ribbon section of relatively hard and resilient silver provided with series necks and connected in series with said wire section, said ribbon section being folded in transverse direction at a plurality of equidistantly spaced points to form a zig-zag-shaped extension spring having in unloaded condition a shorter length than the said predetermined axial-spacing minus the length of said straight wire section and said extension spring being stretched and loaded to such an extent that the length thereof equals said predetermined axial-spacing minus the length of said straight wire section.

9. A highvoltage fuse as specified in claim 7 wherein said composite fusible element structure includes a straight wire section comprising a plurality of parallel-connected wires each supporting a globule of low-fusing point alloy-forming metal adjacent the center thereof, wherein said ribbon section includes a pair of ribbon subsections of unequal length each arranged between said wire section and one of said pair of terminal elements, each of said pair of ribbon subsections being folded in transverse direction at a plurality of equidistantly spaced points to form a pair of zig-zag-shaped extension springs having in unloaded condition an aggregate length less than said predetermined length minus the length of said straight wire section, and said pair of springs being stretched and loaded to such an extent that the aggregate length thereof equals said predetermined axial spacing minus the length of said straight wire section. 

1. A high-voltage fuse including a. a tubular casing of electric insulating material; b. a pair of electroconductive terminal elements closing the ends of said casing; c. a pulverulent arc-quenching filler inside said casing; and d. a composite fusible element structure arranged inside said casing, conductively interconnecting said pair of terminal elements and submersed in said arc-quenching filler, said composite fusible element structure having a straight wire section of circular cross section and a ribbon section of relatively hard and resilient silver provided with series necks and connected in series with said straight wire section, said ribbon section being folded in transverse direction at a plurality of equidistantly spaced points and forming a zig-zagshaped extension spring having a predetermined length in nonloaded condition and an increased length in loaded condition, said extension spring being arranged in loaded condition between said pair of terminal elements and being self-supporting and tending to maintain said straight wire section under axial stress.
 2. A high-voltage fuse as specified in claim 1 including a plurality of composite fusible element structures arranged in a substantially cylindrical pattern around the axis of said casing.
 3. A high-voltage fuse as specified in claim 1 wherein said ribbon section is formed by a pair of aligned ribbon subsections spaced in a direction longitudinally of said casing, wherein said straight wire section is of silver and arranged between said pair of ribbon subsections with the axially outer ends thereof affixed to the axially inner ends of said pair of ribbon subsections, said pair of ribbon subsections jointly tending to maintain said straight wire section under axial stress, wherein said straight wire section is provided with a pair of low-fusing point silver-severing means each adjacent one of said ends thereof affixed to said pair of ribbon subsections, and wherein said straight wire section is provided with an additional low fusing point silver severing means affixed to said straight wire section substantially midway between the ends thereof.
 4. A high-voltage fuse as specified in claim 1 including a pair of composite fusible element structure, the ribbon section of each of said pair of fusible element structures comprising a pair of aligned ribbon subsections of unequal length spaced in a direction longitudinally of said casing, said straight wire section of each of said pair of composite fusible element structures being of equal length and arranged between said pair of ribbon subsections thereof, one relatively short and one relatively long of said ribbon subsections of said pair of composite fusible element structures being arranged immediately adjacent to one of said pair of terminal elements and the other relatively short and the other relatively long of said ribbon subsections of said pair of composite fusible element structures being arranged immediately adjacent to the other of said pair of terminal elements so that said straight wire section of one of said pair of composite fusible element structures is displaced relative to the said straight wire section of the other of said pair of fusible element structures in a direction longitudinally of said casing.
 5. A high-voltage fuse as specified in claim 1 including: a. a straight wire section formed by a plurality of silver wires connected in parallel and each having a deposit of a low-fusing point silver-severing metal adjacent the center thereof; and b. a ribbon section formed by a pair of aligned ribbon subsections spaced from each other in a direction longitudinally of said casing and conductively interconnected by said plurality of silver wires, each of said pair of ribbon subsections having equidistantly spaced circular perforations, being folded at each of said plurality of perforations in transverse direction and stretched to form a pair of zig-zag-shaped loaded springs having planar elements enclosing an angle in the order of 90 degrees.
 6. A high-voltage fuse as specified in claim 1 wherein said straight wire section is formed by a plurality of silver wires connected in parallel, each having a deposit thereon of a low-fusing point silver-severing metal arranged substantially midway between the ends thereof, and said plurality of silver wires including at least three silver wires.
 7. A high-voltage fuse including: a. a tubular casing of electric insulating material; b. a pair of electroconductive terminal plugs press-fitted into the ends of said casing and closing said ends, each of said pair of terminal plugs having grooves in the axially inner end surfaces thereof; c. an arc-quenching filler of quartz sand inside said casing; and d. a plurality of composite fusible element structures arranged inside said casing and submersed in said arc-quenching filler, each of said plurality of composite fusible element structures having an axially inner wire section formed by a plurality of silver wires connected in parallel and each provided adjacent the center thereof with a deposit of a low-fusing point silver-severing metal, said axially inner wire section of some of said plurality of composite fusible element structures being displaced in a direction longitudinally of said casing relative to said axially inner wire section of others of said plurality of composite fusible element structures, each of said plurality of composite fusible element structures further having a pair of aligned axially outer ribbon sections spaced in a direction longitudinally of said casing and conductively interconnected by said axially inner wire section, each of said axially outer ribbon sections being of relatively hard and resilient silver and folded in transverse direction at a plurality of equidistantly spaced points and forming zig-zag-shaped extension springs having a predetermined length when unloaded and an increased length when in axially loaded condition, said extension springs being arranged in axially loaded condition between said pair of terminal plugs and the axially outer ends of said extension springs being inserted into said grooves in said axially inner end surfaces of said pair of terminal plugs and maintained in position therein by solder joints.
 8. A high-voltage fuse including: a. a tubular casing of electric-insulating material; b. a pair of electroconductive teRminal elements closing the ends of said casing and having axially inner surfaces having a predetermined axial spring; c. a pulverulent arc-quenching filler inside said casing; and d. a composite fusible element structure inside said casing, conductively interconnecting said pair of terminal elements and submersed in said arc-quenching filler, said fusible element structure having a straight wire section and a ribbon section of relatively hard and resilient silver provided with series necks and connected in series with said wire section, said ribbon section being folded in transverse direction at a plurality of equidistantly spaced points to form a zig-zag-shaped extension spring having in unloaded condition a shorter length than the said predetermined axial-spacing minus the length of said straight wire section and said extension spring being stretched and loaded to such an extent that the length thereof equals said predetermined axial-spacing minus the length of said straight wire section.
 9. A high-voltage fuse as specified in claim 7 wherein said composite fusible element structure includes a straight wire section comprising a plurality of parallel-connected wires each supporting a globule of low-fusing point alloy-forming metal adjacent the center thereof, wherein said ribbon section includes a pair of ribbon subsections of unequal length each arranged between said wire section and one of said pair of terminal elements, each of said pair of ribbon subsections being folded in transverse direction at a plurality of equidistantly spaced points to form a pair of zig-zag-shaped extension springs having in unloaded condition an aggregate length less than said predetermined length minus the length of said straight wire section, and said pair of springs being stretched and loaded to such an extent that the aggregate length thereof equals said predetermined axial spacing minus the length of said straight wire section. 